The instant invention relates to a process for determining a safety margin with respect to adhesion between the tires and the roadway for motor vehicles during the dynamics of vehicle movement.
Even with all the progress achieved until now in safety technology for motor vehicles, the sudden deterioration of adhesion between tires and roadway which surprises the driver, or is not noticed in sufficient time, still represents a great safety gap. The danger of misjudgments or surprise exists not only when braking, but primarily when entering into curves where even a minor decrease of frictional adhesion to the roadway can have serious consequences.
This danger is even greater with all-wheel drive vehicles which give the driver a subjective feeling of safety. Furthermore, the transition between safety and danger zones is relatively narrower for such vehicles than for conventional vehicles.
Continued progress in safety technology therefore requires that the driver be given a qualified warning as early as possible when the adhesion begins to decrease to a dangerous level. In this case, "qualified" means above all that the warning should reveal how far away the vehicle still is from a danger level.
A process for determining this safety margin is known from DE-C 40 10 507. In this patent document, a characteristic curve is derived and stored from value pairs of the wheel slip and a simultaneously acting peripheral force during stationary or quasi-stationary operating states on a dry roadway while assuming a constant value for the coefficient of friction .mu.. If later value pairs occur which are not located on this characteristic curve, the conclusion is drawn that adhesion has deteriorated and that a different value for the coefficient of friction .mu. applies, i.e., one which represents a rise in the slope of the curve determined by the new value pair.
To speak of a coefficient of friction value .mu. between tires and roadway in connection with load transmission is generally controversial because it is not constant and because this is a very complex process which has little to do with the friction between rigid bodies described by Coulomb's theory. Although only the approximately linear micro-area of the slip curve is used in this known process, there still remains the question how an adhesion value can be determined in the limit zone of the slip curve which is in no case linear.
Furthermore, it may be viewed as disadvantages of this known process that a measurement of the peripheral force on the wheel is required, necessitating intervention into the drive train, and that a wheel slip signal (indicating the difference between the rotational speeds of the driven and non-driven wheels) is required, so that the process cannot be used for all-wheel drive vehicles, for instance. The advantage of continuous detection, even in unchanging travel, must also be weighed against the fact that this evaluation of an adhesion value only takes into account the peripheral elasticity of the tire (i.e., takes into account only tire slip during acceleration or deceleration), without taking into account the transverse elasticity of the tires which is lower and is therefore easier to measure and more indicative than peripheral elasticity.
The situation is similar with the process known from DE-A 37 35 673 where the circumferential acceleration of the driven wheels is measured by using a differential method and is converted by computation into a frictional value .mu. while taking into account constructive magnitudes of the drive train.
Finally, DE-A 36 44 139 teaches a process which compares the acceleration calculated from the rotational wheel speeds with a longitudinal acceleration measured by an acceleration sensor attached to the vehicle. However, this is done only for a plausibility control of the input signals for ABS or ASR systems.
It is therefore the object of the instant invention to propose a process making it possible to detect the safety margin for a vehicle's adhesion to the roadway while taking into account all driving conditions and actually attainable limit zones, which process requires no intervention into the drive train, and which is equally suited for conventional vehicles and all-wheel drive vehicles.